It is known in the prior art to resolve various light gas streams containing hydrocarbons into a purified light gas stream as one product of the resolution and a heavier hydrocarbon stream such as a C.sub.2, C.sub.3, C.sub.4+ or combinations thereof as another product of the resolution. It is also known in the prior art to perfect this resolution with various combinations of low temperature separatory process steps including phase separation, rectification and the use of refluxing heat exchangers sometimes referred to as dephlegmators. It is specifically known to use a combination of low temperature cooling of a feed gas stream followed by phase separation and subsequent dephlegmation to resolve a light gas stream and a heavy hydrocarbon stream from the feed. The heavy hydrocarbon stream can be further resolved in a separatory column.
For example, in UK published patent application GB No. 2146751A published Apr. 24, 1985, to Tomlinson and Cummings, the recovery of LPG or NGL and sales gas from a gaseous hydrocarbon feed is conducted by cooling the feed against process streams and/or refrigeration before phase separating into a light gas and heavy liquid stream followed by dephlegmation of the light gas stream and column stripping of the heavy liquid stream with recycle of the overhead from the column back to feed.
It is also known to perform phase separation and dephlegmation to recover C.sub.2+ hydrocarbons from a feed gas stream containing light gas components as set forth in U.S. Pat. No. 4,002,042. In that process, feed gas is cooled, phase separated, and the light gas is resolved using dephlegmation with refrigeration from an outside external source and by expanding the light gas stream. The heavy liquid stream from the phase separator is resolved in a rectification column using condenser and reboil duty.
Another prior art teaching of dephlegmation, phase separation and fractionating column processing is set forth in U.S. Pat. No. 4,519,825 wherein C.sub.4+ hydrocarbons are separated from the feed gas, particularly wherein the dephlegmator operates by refrigeration produced in expanding the light gas product stream to lower temperature and pressure through an expander.
Other prior art having only general relevance to the technology of the present invention wherein analogous feed streams are resolved into light and heavy components by individual or subcombinations of low temperature cooling, phase separation, column separation or dephlegmation are represented in the following art: D. H. MacKenzie and S. T. Donnelly, "Mixed Refrigerants Proven Efficient in Natural Gas Liquids Recovery Process", Oil and Gas Journal Mar. 4, 1985, page 116; T. R. Tomlinson and R. Banks, "LPG Extraction Process Cuts Energy Needs", Oil and Gas Journal July 15, 1985, Page 81; U.S. Pat. Nos. 3,929,438; 4,270,940; 4,272,269; 4,272,270; 4,303,427; 4,356,014; 4,401,450; 4,443,238; 4,456,460; 4,461,634; 4,482,369; 4,507,133 and 4,526,596.